1. Field of the Invention
This invention relates to an absorption type multi-layer film ND filter which attenuates transmitted light in the visible spectral range. More particularly, it relates to an improvement of an absorption type multi-layer film ND filter making use of a thin substrate such as a resin film, a resin sheet or a thin glass sheet.
2. Description of the Related Art
As ND filters (neutral density filters) of this type, a reflection type ND filter which reflects incident light to attenuate it and an absorption type ND filter which absorbs incident light to attenuate it are known in the art. Then, where an ND filter is set in a lens optical system in which the reflected light comes into question, the absorption type ND filter is commonly used. This absorption type ND filter includes a type in which the substrate itself is incorporated with an absorptive substance (a color glass ND filter) or coated with it, and a type in which the substrate itself does not absorb light and a thin film formed on its surface does. Also, in the case of the latter, in order to prevent reflection at the thin-film surface, the thin film is constituted of a multiple layer so that it can have the function to attenuate transmitted light and also have the effect of preventing reflection.
As an absorption type multi-layer film ND filter in which such a thin film is constituted of a multiple layer, a multi-layer film formed of combination of a dielectric layer and a titanium oxide layer and a multi-layer film formed of combination of a dielectric film layer and a niobium film layer are disclosed in Japanese Patent No. 3359114 and Japanese Patent Application Laid-open No. 2002-350610, respectively.
In Japanese Patent Application Laid-open No. H10-133253, further disclosed is a technique in which a hard film layer of a dielectric is formed on at least one outermost surface so as not to be scratched because of some mechanical contact.
Now, in an absorption type multi-layer film ND filter used in compact thin-type digital cameras, its substrate itself must be thin because the camera has a narrow set-in space, and a very thin glass sheet or resin sheet or a resin film is used as the substrate.
However, where such a very thin glass sheet or resin sheet or a resin film is used as the substrate, a problem peculiar thereto as shown below may come about.
That is, the problem is that, if a multi-layer film having a different structure is formed for each side of the very thin glass sheet or resin sheet or the resin film the substrate may warp toward one side because of the film stress of each multi-layer film formed thereon. In such a case, the substrate does not warp even when the multi-layer film is formed on each side, as long as films are formed under ideal conditions that the overall film stress is cancelled for each one-side multi-layer film, i.e., under conditions such that the substrate does not warp even when the film is formed on the former's one side only. Then, as methods by which the overall film stress is cancelled, available are a method in which the film stress is made not to exist for each layer and a method in which a film with tensile stress and a film with compressive stress are alternately formed to cancel the film stress. It, however is difficult in fact to make zero the film stress of a multi-layer film.
For example, in the case of a commonly available absorption type multi-layer film ND filter in which, as shown in FIG. 1, an absorption type multi-layer film 1 is formed on one side of a substrate 3 and a reflection preventive film 2 is formed on the other side, the substrate may warp if the absorption type multi-layer film 1 and the reflection preventive film 2 are poorly balanced in film stress and where the substrate is constituted of a very thin glass sheet or resin sheet or a resin film. Then, in the case of the absorption type multi-layer film 1 shown in FIG. 1, in which metallic films are so thin as to be almost constituted of SiO2, the above method in which a film with tensile stress and a film with compressive stress are alternately formed to cancel the film stress can not be employed because the metallic films are thin, and the film stress of the SiO2 film must be controlled to be zero. However, it is difficult in fact to set film forming conditions under which the film stress of the SiO2 film can be made zero.
There is also a problem that, if the substrate has warped, it is difficult to handle such an ND filter when it is bonded or welded, and also a difficulty that images may come distorted when it is used in the vicinity of an imaging element is also a matter of concern.